1. Field of the Invention
This invention relates to lubricating oil compositions, their method of preparation, and use. More specifically this invention relates to lubricating oil compositions which contain a molybdenum compound and a secondary diarylamine wherein the molybdenum compound is substantially free of active sulfur. The use of both the molybdenum and amine within certain concentrations provide improved oxidation control and friction modifier performance to lubricating oil compositions. The lubricating oil compositions of this invention are particularly useful as crankcase lubricants.
2. Description of the Related Art
Lubricating oils as used in the internal combustion engines of automobiles or trucks are subjected to a demanding environment during use. This environment results in the oil suffering oxidation which is catalyzed by the presence of impurities in the oil such as iron compounds and is also promoted by the elevated temperatures of the oil during use. This oxidation of lubrication oils during use is usually controlled to some extent by the use of antioxidant additives which may extend the useful life of the oil, particularly by reducing or preventing unacceptable viscosity increases.
We have now discovered that a combination of about 100 to 450 parts per million (ppm) of molybdenum from an oil soluble molybdenum compound which is substantially free of active sulfur and about 750 to 5,000 ppm of an oil soluble secondary diarylamine is highly effective in inhibiting oxidation in lubricant compositions and that this antioxidant performance is supplemented by improved friction modifier performance. The molybdenum acts synergistically with secondary diarylamines to provide significant improvement in oxidation control. In addition to excellent oxidation control, the molybdenum compounds also act as friction modifiers to provide substantial fuel economy performance.
Lubricant compositions containing various molybdenum compounds and aromatic amines have been used in lubricating oils. Such compositions include active sulfur or phosphorus as part of the molybdenum compound, use additional metallic additives, various amine additives which are different from those used in this invention, and/or have concentrations of molybdenum and amine which do not show the synergistic results obtained by this invention.
U.S. Pat. No. 3,285,942 of Nov. 15, 1966 to Esso discloses the preparation of glycol molybdate complexes which have utility in lubrication oils.
U.S. Pat. No. 4,394,279 of Jul. 19, 1983 to L. de Vries et al. discloses an antioxidant additive combination for lubrication oils prepared by combining (a) an active sulfur containing molybdenum compound prepared by reacting an acidic molybdenum compound, a basic nitrogen compound and carbon disulfide with (b) an aromatic amine compound.
U.S. Pat. No. 4,832,857 of May 23, 1989 to Amoco Corp discloses a process for preparation of overbased molybdenum alkaline earth metal and alkali metal dispersions for use in lubricating oil compositions.
U.S. Pat. No. 4,846,983 of Jul. 11, 1989 to W. C. Ward discloses molybdenum containing hydrocarbyl dithiocarbamates prepared from primary amines that impart anti-wear, antioxidant, extreme pressure, and friction properties to lubricating oils. Again, among other shortcomings, these molybdenum compounds contain substantial quantities of active sulfur.
U.S. Pat. No. 4,889,647 of Dec. 26, 1989 to R. T. Vanderbilt Co. discloses organic molybdenum complexes for use in lubrication oil compositions.
U.S. Pat. No. 5,137,647 of Aug. 11, 1992 to R. T. Vanderbilt Co. discloses molybdenum complexes for use in fuels and lubricating oil compositions.
U.S. Pat. No. 5,143,633 of Sept. 1, 1992 to Gallo et al discloses superbasic additives for lubricant oils containing an organic molybdenum complex.
WO95/07962 of Mar. 23, 1995 to A. Richie et al. discloses a crankcase lubricant composition for use in automobile or truck engines which contains copper, molybdenum, and aromatic amines. In addition to the requirement for use of copper, this publication recites a very broad range of concentrations for the molybdenum and the amine whereas the concentrations of amine used with the molybdenum in the examples of that publication is well outside the range which this invention has found to be synergistic. Also, many of the molybdenum compounds of this reference contain active sulphur, phosphorus, and other elements and the amines include compounds such as primary amines which were not found synergistic with the molybdenum carboxylates of this invention.
WO95/07963 of 23 Mar. 1995 to H. Shaub discloses highly sulfurized molybdenum compounds and various secondary aromatic amines having at least one aromatic group for producing a synergistic antioxidant effect when used as an antioxidant additive for lubricating oils. Again the molybdenum compounds contain active sulfur.
WO95/07966 of 23 Mar. 1995 to J. Atherton et al. discloses engine oil lubricants of various molybdenum compounds including that of some with active sulfur, certain organo-phosphorus compounds, an aminic antioxidant and a phenolic antioxidant within certain proportions.